نشریه دانش بیماری شناسی گیاهی
سال سوم شماره 2 (پیاپی 6، بهار و تابستان1393)

Plant-parasitic nematodes are one of the most important pests worldwide and cause considerable economic loss to many of agricultural products. Some of soil inhabited nematodes are affected by some of antagonistic bacteria, so they can be used in biological control. Nematodes can be affected by bacteria in different ways such as direct suppression, promotion of plant growth, and facilitation of rhizosphere colonization. In overall, regarding to effect of soil inhabits bacteria on nematodes; they can be classified as toxin producing, antibiotic producing and enzyme producing as well as plant growth promoting groups. Based on the recent researches, bacteria are divided to six groups including: parasitic bacteria (nematophagous bacteria), opportunistic parasitic bacteria, rhizobacteria, endophytic bacteria, symbionts of entomopathogenic nematodes and cry protein-forming bacteria. Combination of bacteria with some other antagonistic microorganisms was successful in control of plant parasitic nematodes.

In addition to the full-length viral DNA genome, various types of smaller specific DNA molecules have been isolated from plants infected by DNA viruses. These DNAs are usually derived from viral genomes by different ways or have non-viral genome sequences. Some of these DNA have no significant effect on the virus cycle and on the incidence and progression of the disease, while some of them inducing the viral disease symptoms. These components that are known as satellite, defective and defective interfering DNAs, depend on helper viruses for replication, encapsidation and movement in plants. Satellites have no significant homology with the helper virus genome. However, they are required for inducing disease symptoms. While defective and defective interfering DNAs exhibit high homology with the genome of helper viruses, only defective interfering DNAs have ability to interfere with virus replication and with disease symptom induction and development. In this paper, the characteristics of these subviral DNAs and the possible mechanisms by which they are generated and transmitted in virus infected plants are discussed.

Food production and food security is an essential precursor to sustainable development in agriculture. Currently, more than 800 million people, generally in Africa and Asia, suffer from hunger and agriculture is considered as the main source of food for them. One of the application of nuclear technology is reducing the damages of plant pest and diseases. The application of nuclear techniques in plant pathology can be grouped in three categories including disease tracing, mutagenesis induction and radiation of crops to induce resistance and destruction of pathogens. As a new method to induce defense responses to biotic and abiotic stresses, nowadays, gamma radiation is used to improve the growth in the way to induce the plant resistance to environmental tensions and plant pathogens as well. Use of this potential, especially in management of seed and seedling diseases is very important to reduce a big portion of crop losses caused by plant pathogens in the first weeks of seedling growth.

Abiotic stresses are major environmental factors that affect agricultural productivity worldwide. Depending on the crop, the yield losses associated with abiotic stresses can reach 50 to 82 percent. Extreme temperatures, drought, salinity, flooding, freezing, ultraviolet light, heavy metals, nutrient deficiency, unsuitable pH, air pollutants and mechanical damage are the most basic stressors. Because biotic stresses cause metabolic toxicity, membrane degradation, reduction of photosynthesis, decrease of nutrient uptake, changes in levels of phytohormones and ultimately affect the plant growth and its productivity, therefore reducing the effect of these stresses, is essential. Plant growth promoting rhizobacteria play an important role in plant disease management and have a high potential in alleviation the abiotic stresses.

Phytoplasmas are some plant pathogens that establish and propagate in plant phloems. They have transmitted by sucking insects. Phytoplasmas have a different lifecycle as compare to bacterial pathogens. They have ability to infect different hosts two different kingdoms, planta and animalia (insects). They systemically infect their hosts. Phytoplasmas have various approaches for adaptation to their hosts. Some of adaptation mechanisms include: changes in the level of gene expression, variation and recombination in extrachromosomal DNA and potential mobile units, production of effectors and suppression of defense signaling pathways. These approaches enable them to establish, propagate and infect various hosts. Recognizing these strategies would be a major step on the effective management of these pathogens.

Downy mildew, caused by Pseudoperonospora cubensis, is the most important disease of cucumber production in humid areas and greenhouses in Iran. The yield loss of cucumber due to downy mildew may reach to 100%. Because the chemical control is the main method of disease control, especially in susceptible cultivars, establishing a forecasting program which is based on estimation of pathogen population, recording the temperature and relative humidity of field and greenhouse, can reduce the risk of disease epidemic. So by proper application of a protective or systemic fungicide can reduce the disease incidence, production costs and the most important, the amount of fungicide residue.